This invention relates generally to control of bolt tension and in particular to accurate control of bolt tension in bolted joints, for example those used during wheel mounting, and is more particularly directed toward control of bolt tension in wheel mounting applications through accurate control of bolt elongation to produce the desired tension. The invention may also be accepted for other applications such as tensioning cables etc.
Bolt tension is the primary parameter of interest in arrangements where mating nuts and bolts are used to secure mechanical assemblies in position. Although a torque wrench is often the implement prescribed by manufacturers for setting bolt tension, a simple measurement of how tightly the nut and bold are connected does not necessarily translate into an accurate measure of the bolt tension. This discrepancy can occur because of friction between the nut and bolt, or because of dirt or other impurities that may insinuate themselves between the nut and its mating surface.
In the attachment of wheels to vehicles. Traditionally the tension has been achieved by rotation of a nut on a bolt. The rotation pulls the bolt through the nut. Incorrect bolt tension can be particularly dangerous, since it can lead to early failure of the bolts and detachment of the wheel. This failure scenario can be caused by bolt tension being too little, which causes the wheel/hub interface to slip, shearing the bolts. Failure can also be caused by excessive tension, which causes metal fatigue in the bolt and permanent plastic deformation or stretching. This in turn leads to insufficient tension in the system, causing subsequent slippage and shearing of the bolts. Using the traditional methods it is difficult to be confident that the correct tension has been achieved.
Since there is often poor correlation between the amount of torque applied to a nut and the resulting tension in the bolt, a need arises for a new arrangement that allows simple and accurate control of bolt tension, particularly in critical applications such as the mounting of wheels to vehicles.
These needs and others are satisfied by the arrangement of the present invention, in which a bolt tension adjustment may be built into a component, such as a wheel, in a system requiring precise bolt tensioning. In the arrangement described, a wheel may be mounted to a vehicle in such a way that the installer will have a high confidence that the mounting is secure, by virtue of precise setting of bolt tension. This arrangement obviates the uncertainty inherent in installation procedures in which simple torque wrenches are used to set bolt tension. As has been suggested, the simple adjustment of tightening torque does not necessarily correlate well with proper bolt tension in the completed assembly.
To accomplish a proper adjustment, an arrangement as contemplated herein is made a part of the wheel, for example. The arrangement includes cams that are disposed on either side of an opening in the wheel that is designed to accommodate a bolt. The cams are eccentric, as is typical of cam design, and are rotatably mounted on shafts that allow the outer contoured surfaces of the cams to rotate such as to present an increasing cam diameter between an abutting base surface and the nut that engages the mounting bolt. A removable lever assembly allows the installer to rotate the cams to a position of minimum tension, mount the wheel in position so that the mounting bolts protrude properly through the mounting holes, then engage the nuts on the mounting bolts, hand tightening them against an abutting surface. The lever is then used to rotate the cams into a second position that presents the cams"" maximum diameter between the base surface and the nut. This positioning of the cams elongates the bolts to the point where bolt tension is optimum and a secure coupling of bolts and nuts is ensured. The lever assembly is then withdrawn to prevent tampering.
In one form of the invention, a tension control arrangement including a base having an opening therethrough to accommodate a bolt and a nut in engagement with the bolt is provided. This tension control arrangement is characterised by a plurality of cams disposed about the opening, each of the cams having a contoured outer surface, and wherein each of the cams is rotatably supported such that the contoured outer surface operatively engages both the base and a contact member interposed between the cam and the nut. A lever assembly is operatively coupled to the cams, such that movement of the cams is substantially restricted to rotational motion from a first position in which the nut is engaged with the bolt and tightened by hand until it is seated against the contact member, to a second position in which the bolt is elongated to achieve a predetermined bolt tension. One or more grooves may be provided in the base, each of the grooves having an arcuate inner surface upon which the contoured outer surface of each cam rests.
The contact member interposed between the cam and the nut is a pressure block having a groove facing the contoured outer surface of the cam, the groove having an arcuate inner surface upon which the contoured outer surface of the cam rests. In one embodiment, the contact member interposed between the cam and the nut has an opening formed therethrough to accommodate the bolt, and the contact member is integrally formed so as to provide contact members for a pair of cams. The lever assembly that is operatively coupled to the cams includes elements that contact the base assembly to substantially preclude cam rotation except within a range between the first position and the second position.